Carbon Composites with Improved Tensile Properties

Accepted Manuscript Title: A Novel Multiscale Reinforcement by in-Situ Growing Carbon Nanotubes on Graphene Oxide Grafted Carbon Fibers and Its Reinforced Carbon/Carbon Composites with Improved Tensile Properties Author: Yunyu Li, Ling-jun Guo, Ya-wen Wang, He-jun Li, Qiang Song PII: DOI: Reference:

S1005-0302(15)00237-6 http://dx.doi.org/doi: 10.1016/j.jmst.2015.12.022 JMST 631

To appear in:

Journal of Materials Science & Technology

Received date: Revised date: Accepted date:

2-8-2015 11-11-2015 23-11-2015

Please cite this article as: Yunyu Li, Ling-jun Guo, Ya-wen Wang, He-jun Li, Qiang Song, A Novel Multiscale Reinforcement by in-Situ Growing Carbon Nanotubes on Graphene Oxide Grafted Carbon Fibers and Its Reinforced Carbon/Carbon Composites with Improved Tensile Properties, Journal of Materials Science & Technology (2015), http://dx.doi.org/doi: 10.1016/j.jmst.2015.12.022. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

A Novel Multiscale Reinforcement by In-situ Growing Carbon Nanotubes on Graphene Oxide Grafted Carbon Fibers and Its Reinforced Carbon/carbon Composites with Improved Tensile Properties Yunyu Li 1, Ling-jun Guo 1, Ya-wen Wang 1, He-jun Li 1, Qiang Song 1,2,*

1

State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an 710072, China

2

Center for Nano Energy Materials, Northwestern Polytechnical University, Xi'an 710072, China

[Received 2 August 2015; Received in revised form 11 November 2015; Accepted 23 November 2015] * Corresponding author. Ph.D.; Tel.: +86 29 88495764; Fax: +86 29 88495764.

E-mail address: [email protected] (Q. Song).

In-situ growing carbon nanotubes (CNTs) directly on carbon fibers (CFs) always lead to a degraded tensile strength of CFs and then a poor fiber-dominated mechanical property of carbon/carbon composites (C/Cs). To solve this issue, here, a novel carbon fiber-based multiscale reinforcement is reported. To synthesize it, carbon fibers (CFs) have been firstly grafted by graphene oxide (GO) and then carbon nanotubes (CNTs) have been in-situ grown on GOgrafted CFs by catalytic chemical vapor deposition. Characterizations on this novel reinforcement show that GO grafting cannot only nondestructively improve the surface chemical activity of CFs but also protect CFs against the high-temperature corrosion of metal catalyst during CNT growth that maintain their tensile properties. Tensile property tests for unidirectional C/Cs with different preforms show that this novel reinforcement can endow C/C with improved tensile properties, 32% and 87% higher than that of pure C/C and C/C only doped with in-situ grown CNTs. This work would open up a possibility to 1

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fabricate multiscale C/Cs with excellent global performance.

Key words: Multiscale reinforcement; Carbon fiber; Graphene oxide; Carbon nanotube; Carbon/carbon composite; Tensile property 1. Introduction Carbon-based multiscale composites, consisting of nanoscale carbon nanotube (CNT), microscale carbon fiber (CF) and pyrocarbon (PyC) matrix, have stimulated tremendous interest in the development of high performance carbon/carbon composites (C/Cs) since the introduction of CNTs can offer great improvements in compressive [1], interlaminar[2], thermal-conductive [3], frictional properties[4] , and so on[5,6] . The preparation of CNT doped CF hybrid reinforcement (or preform) is the first step to obtain this new type composite. So far, most researches have been done for preparing the hybrid preforms through in-situ growth method by catalyst chemical vapor deposition (CCVD) [1–3,5,6] . However, the use of tough chemical treatments for CFs before metal catalyst loading, the metal atoms dissolved into CFs at high temperature, and the additional thermal exposure always lead to serious negative influences on the tensile strength of CFs and the CF-dominated mechanical properties of the composites [7–10]. For example, although the interlaminar shearing strength and compressive strength of C/C composites reinforced by in-situ grown CNTs could have over 200% improvements compared with the baseline, their flexural strength (dominated by the tensile strength of CFs) decreases about 30%[11] . Several studies have been carried out to prevent the degradation of CFs during the CNT growth process, such as lowering the CNT growth temperature below the eutectic temperature [7], using unsized CF [12] , decreasing the CNT density[13] and utilizing the aerosolassisted chemical vapor deposition (CVD) [14] . Although the reduction of the CF strength is attenuated under these conditions, it was not successful enough to change the reduction

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trend. Graphene oxide (GO) has excellent mechanical properties, which has been grafted onto CFs to develop new hybrid reinforcements recently [15,16] . Due to the abundant functional groups, grafting GO onto CFs can nondestructively improve the surface c hemical activity of CFs greatly [15,16] and may also prevent the dissolution of metal catalyst into CFs because GO can help stabilizing small metal particles and avoiding the direct contact of CFs with metal particles, which would endow the CNT-doped CFs with good tensile performance. Based on this consideration, here, we propose a new reinforcement consisting of CF, GO and CNT (denoted as “CNT–GO–CF”) and its positive effect on the tensile performance of unidirectional C/Cs was demonstrated. 2. Experimental The preparation of CNT–GO–CF is described as follows: GO was synthesized from natural graphite by the well-known Hummers method [17]. As-synthesized GO was finally dispersed in water (0.5 mg/mL) and ultrasonically exfoliated for 6 h in an ultrasonic bath (300 kW, 45 kHz), which was used to obtain GO aqueous suspension. CF tows (T300, 3k, fiber diameter: 6–7 μm, Tianniao, China) were immersed in 65 wt% heavy HNO3, which was used to remove the sizing and also to introduce some carboxyl groups onto CFs. To avoid the surface damage of CFs, the immersion was carried out at room temperature only for 1.5 h [18]. Desized CF tows were impregnated with GO aqueous suspension for 12 h with the help of vacuum. After that, the hybrid tows were taken out and annealed at 150 °C for 1 h, which was used to afford covalent bonding between the GOs and CFs [16]. And then, GOgrafted CF tows were impregnated with 1.0 wt% Ni(NO 3 )2·7H2O acetone solution for 5 s and then dried in air. They were loaded into a CVD reactor (inner Ø = 40 mm, length = 1200 mm) and calcinated in flowing N 2 (75 L/h) at 400 °C for 15 min to decompose the nickel salt into its corresponding oxide and then was reduced in situ by flowing H 2 (120 L/h) at 450 °C for 10 min. After that, the samples were heated to 760 °C under N 2 atmosphere (75 L/h) for

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CNT growth. C 3 H6 was fed at a flow rate of 8 L/h as carbon source; meanwhile, H 2 was also fed at 120 L/h. The growth time was 20 min. Different kinds of CF preforms, including pristine desized CF, CF doped only by in-situ grown CNTs (denoted as “CNT–CF”), and CNT–GO–CF, were densified by 10 h chemical vapor deposition (CVD) at 1100 °C using CH4 as carbon source, which was used to fabricate different kinds of unidirectional C/Cs, i.e. pure C/C, CNT–C/C, and CNT–GO–C/C. GO, GO-grafted CF (GO–CF), and pure CF were characterized by X-ray photoelectron spectroscopy (XPS) (Axis-Nova, Kratos Analytical Ltd. UK). The microstructure and morphology of the samples were investigated by transition electron microscopy (TEM, FEI Tecnai F30G 2 ) and scanning electron microscopy (SEM, FEI NANO450). To evaluate the influence of GO-grafting and CNT growth on the tensile properties of CF preforms and the reinforcing role of CNT–GO–CF, the mechanical properties of the CF preforms during different stages and different kinds of unidirectional C/Cs were respectively obtained by uniaxial tension tests, in which the gauge length was 20 mm. Twelve tows were tested for each sample. 3. Results and Discussion Fig. 1(a) shows the TEM image of the GO used for grafting on CFs. Large and transparent GO sheets were observed with wrinkled structures. Selected area electron diffraction pattern (SAED, Fig. 1(c)) reveals that the GO has a microstructure intervening between ideally crystalline graphene and amorphous carbon due to the observation of diffraction spots (shown by arrows) and diffraction rings. Notably, the intensities of all the diffraction spots are not equal and sharp enough; moreover, the diffraction spots are associated with some unresolved spots, indicating the formation of few layer graphene [19] . Raman spectrum of GO (Fig.1(b)) shows that the G band is broadened and shifts to 1593

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cm –1 (pristine graphite displays a prominent G peak as the only feature at 1581 cm –1 ). And, the D band at 1352 cm –1 becomes prominent, indicating the destruction of sp2 character and the formation of defects in the sheets, due to the extensive inserting of oxy gencontaining groups during the oxidation. The surface morphologies of CFs grafted by GO were observed by SEM, as shown in Fig. 2. Fig. 2(a and b) shows that GO sheets are uniformly attached to CFs where many wide and deep grooves are visible along the axial direction of CF, which is quite different from the Li group’s work where lots of GO sheets stably stand on the CF [15] . The extensive attachment, which probably resulted from the chemical bonding between CFs and GOs [16], can improve the surface free energy of CFs more efficiently due to the massive functional groups of GO sheets[15] . Enlarged SEM image shows that the thickness of GO sheets ranges from 10 nm to over 20 nm, i.e. Fig. 2(d) shows a sheet with the thickness of about 15 nm, indicating GO layers self-assembled and overlapped during the dewatering process. The elemental chemical states of grafting process were assessed by XPS. The typical XPS spectra for desized CF, GO and GO-grafted CF were obtained. The raw XPS data for each sample were analyzed to determine peak locations and areas in relation to specific binding energies which could best fit the experimental data. The C1s peak positions (Fig. 3) were derived from peak deconvolution. For the desized CF, as shown in Fig. 3(a), besides the main C–C peak at 284.6 eV that represents graphitic carbon, the additional higher binding energy peaks represent the different elemental chemical states [11,15] . –COOH and – OH groups are identified on the CF surface and their contents are 14.4% and 10.9%, respectively. For GO, –COOH and –OH groups are also found; however, their contents are 2.5% and 48.5%, quite different from that of CFs. After 150 °C annealing for 1 h, some – COOH and –OH groups on the GO and the CF surfaces are expected to react with each other to afford a strong covalent binding, as reported by Chen group [20] and Huang et al. [16]. The 5

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grafting of GO on CFs means the introduce of abundant –OH and other oxygen-containing groups on CFs (Fig. 3(c)), which is essential to the CNT in-situ growth on CFs by CCVD because these oxygen-containing groups can encourage the efficient and homogeneous loading of metal catalysts with small size and then the homogeneous and dense growth of CNTs[2] . The morphology of CNTs grown on GO-grafted CFs was investigated by SEM (Fig. 4). Fig. 4(a) shows the morphology of GO-grafted CFs after 5 min CCVD, where CNTs with a small yield and heterogeneous dispersion have been found. After 20 min CCVD, the CNT growth becomes so dense and homogeneous that the GO-grafted CFs are totally covered (Fig. 4(b)). Notably, from Fig. 4 (a), we can see CNT growth occurs on wrinkled and transparent GO sheets, as labeled by “GO”, rather than CF surface. More detailed eviden ce is shown in TEM images of GO-grafted CFs before (Fig. 5(a and b)) and after CNT growth (Fig. 5(c and d)). Fig. 5(c and d) shows that the CNTs grew on GO sheets by standing and covering on CF, respectively. The CNT growth on GO sheets indicates that there is no direct connection between catalyst particles and CFs that avoids the CF surface damage caused by the solid solution reaction between metal crystal and solid carbon at high temperature. Another importance of GO grafting is to facilitate the dense CNT growth on CFs. Compared with GOgrafted CFs, the CNT growth on desized CFs by 1.5 h concentrated HNO 3 treatment is quite difficult, showing a heterogeneous dispersion and a very limited yield (Fig. 6 (a)), even under the same CCVD conditions and prolonging the impregnation time of desized CFs in acetone solution and increasing the impregnation temperature, which is related to the high chemical inertness of CFs and the resulting poor catalyst loading morphology. To obtain dense and homogeneous CNT growth on pristine CFs, before catalyst loading, it is necessary to use tough chemical treatments for CFs as reported by Qian et al. [2], i.e. acid oxidation (65 wt% HNO 3) for 5 h at 110 °C. After 20 min CCVD, CNTs with expected

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morphologies have been grafted on CFs. However, the direct loading of catalysts on CFs brings about the serious surface corrosion of CFs during CCVD, as shown in Fig. 6 (b) inset that is the SEM image of Ni-loaded CF after 20 min annealing at 700 °C in high-purity (99.999%) N 2 atmosphere, which will impose a negative influence on the CF tensile property. On the other hand, for GO-hybridized CF tows, the CNT dense growth also occurs in the tow which indicates GO sheets can infiltrate into the space between CFs under the vacuum impregnation and facilitate the CNT growth there (Fig. 4(c and d)), ensuring that CNTs provide intra-tow reinforcement. The tensile properties of CF tows undergoing different processes were also tested, as shown in Fig. 7. Compared with pristine CF tows, the tensile strength of GO-grafted CF tows has a 3.4% increase, which should be attributed to the protection of CF tows from GO sheets as promising sizing agent [16] against the damage occurred during the handling process. The tensile strength of CF tows after 20 min 760 °C annealing has a slight decrease (~2%) due to the thermal exposure [12] . The tough acid treatments make tensile performance of the CF tows decreased by about 21%, compared with pristine CF tows, which is mainly related to the abundant damages and defects occurr ed on CFs after acid treatments[11] . After CNT direct growth on acid-treated CFs, the tensile performance further decreases by 31.6%, due to the erosion of Ni atoms into CFs as shown in Fig. 6(b) inset. For GO-grafted CF tows, after CNT growth, the tensile property (ultimate fracture load: 57 N) nearly does not decrease compared with pristine CF tows (ultimate fracture load: 58 N), which is attributed to the protection from GO sheets to CFs. To evaluate the reinforcing role of CNT–GO–CF, tensile properties of unidirectional C/C, CNT–C/C, and CNT–GO–C/C samples were tested. Results show that CNT–GO–C/C sample has an ultimate fracture stress of 502 ± 46 MPa, ~32% and ~87% higher than that of C/C (380 ± 30 MPa) and CNT–C/C (268 ± 33 MPa), respectively. Fig. 8 shows the typical tensile 7

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stress–strain curves of three kinds of composites. According to the increased slope of the linear section, it can be concluded that the intra-tow cohesion of the sample is obviously enhanced after the dope of CNTs, which should be beneficial to the stiffening role of CNT network to PyC matrix [1,2]. On the other hand, for pure C/C, there are some small steps in the curve, indicating gradual debonding between CF and PyC during the fracture, corresponding to a relatively poor fiber/matrix interfacial bonding; while, for CNT –GO–C/C and CNT–C/C, their stress–strain curves have an almost completely linear section before the maximum stress with no obvious steps, exhibiting a brittle fracture feature, which indicates there is no debonding between CF and PyC throughout the fracture process of the sample, corresponding to a strong fiber/matrix interfacial bonding. In the case of CNT–GO– C/C, the stiffened PyC and the increased fiber/matrix interfacial bonding jointly contribute to the improved tensile strength. This conclusion can be confirmed by the fracture surface morphology by SEM (Fig. 9(a)), in which good interfacial bonding and integrated fracture surface of CNT-doped PyC can be seen. By contrast, for CNT–C/C, although similar microstructure improvements exist, the degraded mechanical property of CFs leads to the decreased tensile strength, which is caused by etch pits as shown in Fig. 9(a) inset. For pure C/C, debonded fiber/matrix interface and PyC fragments are the key fracture features (Fig. 9(b)), corresponding to a poor fiber/matrix interfacial bonding and a poor cohesion of PyC matrix, which lead to a relatively low tensile strength. These tensile property comparisons show the obvious preponderance of the hybrid of CNT–GO–CF as advanced reinforcement. 4. Conclusion It has been demonstrated that the graft of GO can nondestructively improve the surface chemical activity of CFs and protect them against the damages caused by the corrosion of metal catalysts during CNT growth, which endows CNT–CF multiscale hybrid reinforcements with excellent tensile properties. Tensile property tests for unidirectional

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C/Cs with different preforms demonstrate that the use of CNT–GO–CF reinforcement can endow C/C with improved tensile properties, 32% and 87% higher than that of pure C/C and C/C only doped with in-situ grown CNTs, respectively, which opens up a possibility to fabricate multiscale C/Cs with excellent global performance. The future work should be paid on the optimizing of the CNT growth morphology on GO-grafted CFs, such as radially aligned with respect to the fiber surface, for maximizing the reinforcing role of CNTs. Acknowledgments This work is supported by the National Natural Science Foundation of China (Nos. 51432008, 51502242, U1435202, and 51202194), the Research Fund for the Doctoral Program of Higher Education of China (No. 20126102110013), and the Key Grant Project of Chinese Ministry of Education (No. 313047).

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Figure captions

Fig. 1. (a) TEM image of GO used for grafting on CFs; (b) Raman spectra of GO and pristine graphite; (c) SAED pattern of the used GO sheets (arrows show the overlapped diffraction spots).

Fig. 2. SEM images of GO sheet grafted on CFs: (a) low magnification image; (b) enlarged SEM image of the area labeled in (a) by a dotted box; (c) GO sheet with a extended branch; (d) enlarged SEM image of the area labeled in (c) by a dotted box, showing GO sheet grafted on CF with the thickness of about 15 nm.

Fig. 3. C1s XPS fine scan spectra of desized CF (a), GO (b) and GO-grafted CFs (c).

Fig. 4. SEM images of CNTs grown on GO-grafted CFs after 5 min (a) and 20 min (b) CCVD; (c) SEM image of CNTs grown in a CF tow grafted by GO sheets; (d) enlarged image of the grown CNTs.

Fig. 5. TEM images of GO-grafted CFs (a) and (b); TEM images of CNTs grown on GOgrafted CFs (c) and (d).

Fig. 6. (a) SEM image of CNTs grown on desized CFs by 1.5 h heavy HNO3 treatment by CCVD; (b) SEM image of CNTs grown on CFs treated by heavy HNO3 for 5 h at 110 °C. The inset in (b) shows the surface morphology of CF after 20 min annealing at 700 °C in high-purity (99.999%) N 2 atmosphere. Fig. 7. Tensile performances of 1k CF tows undergoing different processes.

Fig. 8. Tensile stress–strain curves of three kinds of unidirectional C/Cs.

Fig. 9. SEM images of tensile fracture surfaces of CNT–GO–C/Cs (a) and C/Cs (b).

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Figure list:

Fig. 1. (a) TEM image of GO used for grafting on CFs; (b) Raman spectra of GO and pristine graphite; (c) SAED pattern of the used GO sheets (arrows show the overlapped diffraction spots).

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Fig. 2. SEM images of GO sheet grafted on CFs: (a) low magnification image; (b) enlarged SEM image of the area labeled in (a) by a dotted box; (c) GO sheet with a extended branch; (d) enlarged SEM image of the area labeled in (c) by a dotted box, showing GO sheet grafted on CF with the thickness of about 15 nm.

Fig. 3. C1s XPS fine scan spectra of desized CF (a), GO (b) and GO-grafted CFs (c).

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Fig. 4. SEM images of CNTs grown on GO-grafted CFs after 5 min (a) and 20 min (b) CCVD; (c) SEM image of CNTs grown in a CF tow grafted by GO sheets; (d) enlarged image of the inset of c.

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Fig. 5. TEM images of GO-grafted CFs (a) and (b); TEM images of CNTs grown on GOgrafted CFs (c) and (d).

Fig. 6. (a) SEM image of CNTs grown on desized CFs by 1.5 h heavy HNO3 treatment by CCVD; (b) SEM image of CNTs grown on CFs treated by heavy HNO3 for 5 h at 110 °C. The inset in (b) shows the surface morphology of CF after 20 min annealing at 700 °C in high-purity (99.999%) N 2 atmosphere. 15

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Fig. 7. Tensile performances of 1k CF tows undergoing different processes.

Fig. 8. Tensile stress-strain curves of three kinds of unidirectional C/Cs.

Fig. 9. SEM images of tensile fracture surfaces of CNT–GO–C/Cs (a) and C/Cs (b).

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